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Effect of Pd Thickness on the Interfacial Reaction and Shear Strength in Solder Joints Between Sn-3.0Ag-0.5Cu Solder and Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) Surface Finish

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Abstract

Intermetallic compound formation at the interface between Sn-3.0Ag-0.5Cu (SAC) solders and electroless nickel/electroless palladium/immersion gold (ENEPIG) surface finish and the mechanical strength of the solder joints were investigated at various Pd thicknesses (0 μm to 0.5 μm). The solder joints were fabricated on the ENEPIG surface finish with SAC solder via reflow soldering under various conditions. The (Cu,Ni)6Sn5 phase formed at the SAC/ENEPIG interface after reflow in all samples. When samples were reflowed at 260°C for 5 s, only (Cu,Ni)6Sn5 was observed at the solder interfaces in samples with Pd thicknesses of 0.05 μm or less. However, the (Pd,Ni)Sn4 phase formed on (Cu,Ni)6Sn5 when the Pd thickness increased to 0.1 μm or greater. A thick and continuous (Pd,Ni)Sn4 layer formed over the (Cu,Ni)6Sn5 layer, especially when the Pd thickness was 0.3 μm or greater. High-speed ball shear test results showed that the interfacial strengths of the SAC/ENEPIG solder joints decreased under high strain rate due to weak interfacial fracture between (Pd,Ni)Sn4 and (Cu,Ni)6Sn5 interfaces when the Pd thickness was greater than 0.3 μm. In the samples reflowed at 260°C for 20 s, only (Cu,Ni)6Sn5 formed at the solder interfaces and the (Pd,Ni)Sn4 phase was not observed in the solder interfaces, regardless of Pd thickness. The shear strength of the SAC/ENIG solder joints was the lowest of the joints, and the mechanical strength of the SAC/ENEPIG solder joints was enhanced as the Pd thickness increased to 0.1 μm and maintained a nearly constant value when the Pd thickness was greater than 0.1 μm. No adverse effect on the shear strength values was observed due to the interfacial fracture between (Pd,Ni)Sn4 and (Cu,Ni)6Sn5 since the (Pd,Ni)Sn4 phase was already separated from the (Cu,Ni)6Sn5 interface. These results indicate that the interfacial microstructures and mechanical strength of solder joints strongly depend on the Pd thickness and reflow conditions.

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Kim, Y.M., Park, JY. & Kim, YH. Effect of Pd Thickness on the Interfacial Reaction and Shear Strength in Solder Joints Between Sn-3.0Ag-0.5Cu Solder and Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) Surface Finish. J. Electron. Mater. 41, 763–773 (2012). https://doi.org/10.1007/s11664-012-1921-0

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